What is Chromatic Aberration? Chromatic aberration arises from dispersion- the property that the refractive index of glass differs with wavelength. There are two types of chromatic aberration: longitudinal aberration, which corresponds to the tracking error, and lateral aberration, which corresponds to the registration error. Longitudinal chromatic aberration: This form of aberration causes different wavelengths to focus on different image planes. It corresponds to the tracking error. In a zoom lens, the amount of the longitudinal chromatic aberration varies as the lens is zoomed. In fig focal length is plotted on the horizontal axis and on-axis chromatic aberration on the vertical axis. The aberration is largest at the telephoto end. If a large longitudinal chromatic aberration is left, tracking error will occur on the blue and red channels and cause colour blurring, even when the tracking is adjusted to optimum. In a long focal length, high zoom ratio lens, chromatic aberration is the greatest problem, particularly the secondary spectrum, which is a high order chromatic aberration. The chromatic aberration of a lens is usually corrected at two wavelengths. The secondary spectrum is the residual chromatic aberration left at the wavelength midway between these two. Two-wavelength correction is inadequate in a television camera that has three (red, blue, and green) channels. The secondary spectrum must also be corrected. The main cause of the problem is the residual chromatic aberration of the focusing group of lenses. It is difficult to solve because of inherent limits in the dispersion (wavelength characteristic of refractive index) of optical glass. The secondary spectrum of Canon lenses is corrected by using fluorite crystal, which has different dispersion characteristics from ordinary optical glass, or by using a type of glass with an extraordinary dispersion characteristic. Lateral chromatic aberration: Lateral chromatic aberration occurs because the magnification of the image differs with wavelength. In a television camera it causes registration error. Lateral chromatic aberration also has a secondary chromatic aberration, making it difficult to correct all three, red, blue and green, wavelengths at the same time. In Fig focal length is plotted on the horizontal axis and lateral chromatic aberration on the vertical axis. Note how the red and blue registration lines tend to cross the green line as they move from wide-angle to telephoto.